Model-free isoconversion methods which use approximations of the temperature integral are generally reliable methods for the calculation of activation energies of thermally activated reactions studied during linear heating. These methods generally neglect the temperature integral at the start of the linear heating, I(T-o). An analytical equation is derived which describes the deviations introduced by this assumption. It is shown that for most reactions encountered this assumption does not have a significant influence on the accuracy of the method. However in cases where T-o is within about 50 to 70 K of the reaction stage to be investigated and activation energies are relatively low, significant deviations are introduced. It is shown that some of the published thermal analysis work on activation energy analysis of reactions occurring at relatively low temperatures is affected by these deviations. Examples are specific cases of dehydration reactions, cure reactions and cluster formation in Al alloys.